The present invention relates to methods and apparatus for controlling the charging of a rechargeable battery and, more particularly, to optimizing the charging or discharging process of such batteries to ensure that full charge is achieved without damaging the battery.
Rechargeable storage cells or batteries are electrochemical devices for storing and retaining an electrical charge and later delivering that charge as useful power. A battery may typically have a number of storage cells connected together to form a battery having specific voltage or current delivery capability. A familiar example of a rechargeable battery is the nickel-cadmium, NiCd, battery used in portable electronic devices such as cameras and mobile telephones.
When charging a rechargeable battery, such as for example an NiCd battery, an electrical voltage greater than the terminal voltage of the battery is applied to the terminals of the battery, so that current flows through the battery. This current initiates a chemical process by which energy is stored in the battery.
When the battery has reached a full charge condition, the chemical process stops and the added energy is instead converted into heat. Because the battery is constructed as a sealed container, the pressure in the battery increases, and tends to cause mechanical and chemical destruction. Chemical destruction may reduce the capacity of the battery, and the capacity may eventually be reduced significantly after several such chargings. To optimize the battery utilization, it is therefore important to ensure not only that the battery will be charged fully, but also that charging is interrupted before the generation of heat becomes too great. Thus, it is desirable to accurately control the charging process to obtain an almost optimum charging and/or to interrupt charging at a proper time.
Accurate control of the charging process is especially important when it is desired to charge the battery as quickly as possible. Frequently, it is desirable that the charging period for a battery (the period of time necessary to charge the battery) be as brief as possible. This has led to the use of greater and greater charging currents. Higher levels of current, however, may tend to accelerate the heat-generating process within the battery. Because the heat-generating process may be accelerated, it is particularly important to interrupt (e.g., terminate) the charging at the proper time. If the charging is interrupted too late, the result is heat generation and mechanical and chemical destruction as mentioned, and if the charging is cut off too early, the battery will be charged only to part of its full capacity.
In a typical charging process, the voltage across the battery initially increases evenly as the battery is charged. However, as the battery approaches its full state of charge, the voltage increases more steeply to a peak (the full state of charge). The voltage then drops again; after the full charge is reached, temperature increases, and the temperature coefficient of the voltage is negative. Correspondingly, the charging current typically falls to a minimum at full charge and then increases.
A number of methods are known which attempt to terminate charging at the proper time. However, they have all been found to be lacking in precision.
For example, one known method comprises measuring the temperature of the battery and cutting off the charging when an increase in temperature is observed. However, by the time the increase in temperature is sufficiently high to measure, it is often too late. It is also difficult to measure the temperature with sufficient accuracy, because among other things, there may be variations in the ambient temperature. This is the case, e.g., if a battery from an automobile telephone is moved in winter from a cold car to a charger which is at room temperature.
Other known methods involve measuring the voltage across the battery or the magnitude of the charging current and terminating charging when the voltage or current assumes a specific level. However, it is very difficult to determine the voltage (or current) at which charging should be terminated. For example, voltage, at full capacity, often varies somewhat from battery to battery, even in case of batteries of the same type. Moreover, the voltage is temperature dependent. Similarly, it is possible to measure the charging current, and the same observations apply here as well.
Many known chargers use a fixed period of time; charging is simply terminated after the elapse of this time. However, the time period necessary to charge the battery is strongly dependent upon the initial charge on the battery (whether the battery is completely or only partially discharged), and the battery charge state. This might be solved by fully discharging the battery prior to the charging. However, such an approach is unsatisfactory, in addition to the waste of energy involved, it takes a certain time, and there will still be a good deal of difference between the necessary charging time from battery to battery.
A more advanced method is to measure the voltage change (or current change) as a function of time, i.e. the slope of a curve showing the voltage as a function of time. For example, U.S. Pat. No. 4,052,656 discloses a method which detects the point at which this slope is zero, corresponding to the peak occurring where the battery is fully charged. However, it is difficult to determine this point accurately since the curve approaching the peak may be very flat. Further, there may be other points on the curve where the slope is zero, resulting in premature termination of charging.
U.S. Pat. No. 4,747,854 describes a system which detects when the voltage curve assumes a negative slope exceeding a reference value. The observations made with respect to zero slope detection also apply here. Moreover, by the time the negative slope occurs, the battery may have already been overcharged to a point sufficient to damage the battery.
U.S. Pat. No. 4,388,582 similarly describes a system which measures the slope of the voltage curve to find the point where the slope of the curve begins to decrease after having been increasing. While the battery will rarely be overcharged using this method, the systems remain susceptible to premature termination of charging; the battery is often only charged to part of its full capacity. Further, there is also risk of incorrect measurements if, e.g., the charging current or the voltage supply is changed during charging.
Use of a combination of various of the above-mentioned methods is also known. For example, U.S. Pat. No. 4,639,655 describes a system which uses four stop criteria, viz. a voltage limit, a predetermined time limit, a calculated increase on the voltage curve, as well as the point where the slope of the voltage curve is zero. The charging is interrupted if any one of these criteria is satisfied. The predetermined time limit is selected after the charging has begun; an initial voltage measurement is made, a short or a long charging time is selected, e.g., 1 hour or 1.75 hours on the basis of the measurement. The advantage of such an approach is that the battery discharge state and the number of cells in the battery can be taken into consideration from the beginning. However, such a system is rather imprecise, and remains susceptible to battery overcharging.